As the demands for microstructures (e.g. micromirror devices) or semiconductor devices (e.g. integrated circuits) with increased performance and low cost increase, the drive for continual reduction in size of microstructures or semiconductor devices constantly increases. Reducing the size, or scale, of the components of a microstructure or a semiconductor device also requires being able to form and pattern components of microstructures on such reduced scales, consistently, robustly, and reproducibly, preferably in a self-aligned manner. In microstructure and semiconductor fabrications, etching steps are important from front-end processes to back-end processes, and are frequently key steps in combination with lithography steps. The scaling of microstructures or semiconductor devices can be limited by physical limits imposed by etching and the subsequent cleaning steps.
For example in making a micromirror device that often comprises a deformable hinge held above a substrate and a reflective mirror plate attached to the deformable hinge, a deep-ultraviolet (hereafter DUV) photolithography process is employed to define components (e.g. the deformable hinge) with critical dimensions. An element with a critical dimension is often referred to as an element has a desired characteristic dimension of sub-micron, such as 1 micron or less, or 0.5 micron or less. Because DUV photoresist, as well as the inorganic bottom anti-reflective coating (BARC) that is typically used in deep ultraviolet (DUV) photolithography, may not be removed through a typical clean process, such as a wet clean process for removing I-line photoresist that is widely used in fabricating microstructures, additional cleaning processes, such as ashing is performed to remove the DUV resist and BARC material. The additional cleaning process, however, may cause unwanted trenches or features, for example, in the spacer layer, which in turn affect the subsequent mirror plate. For demonstration purposes, FIG. 1a and FIG. 1b diagrammatically illustrate unwanted trenches caused by a DUV process in fabricating a deformable hinge of a micromirror device.
Referring to FIG. 1a, deformable hinge 102 of a micromirror device is formed on spacer layer 100 that comprises a sacrificial material. In forming the deformable hinge (102), a hinge layer comprised of a selected hinge material is deposited on spacer layer 100. A hinge mask layer that comprises dielectric layer 103, BARC layer 104, and DUV resist layer 106 is deposited on the hinge layer. In some examples, the hinge mask layer may not comprise a dielectric layer (e.g. dielectric layer 103), and instead, the BARC and DUV resist layers are directly deposited on the hinge layer (or the layer to be patterned). The hinge mask layer is then patterned so as to form a hinge mask with desired features/pattern. The patterned hinge mask layer is then used as the mask for forming (e.g. patterning) the desired features (e.g. the deformable hinge with a critical dimension) in the hinge layer (102).
After forming the features in the hinge layer, the dielectric layer (103), the BARC layer (104), and the DUV resist layer (106) need to be removed by etching. Wet-etching is now widely used for this purpose. However, some wet-etching processes are significantly less effective on BARC layers. Some other wet-etching processes, even though, are capable of removing both of the DUV resist and the BARC layers, but may cause negative impact to the functional members of the device. For example, a wet-etching process may also etch the spacer layer, the hinge layer formed on the spacer layer, and/or the layer below the spacer layer. An approach top this problem is to perform a wet-etching process followed by an ashing process.
During the ashing process, the DUV photoresist and the BARC material are removed. However, the ashing process also etches the exposed spacer layer (100) resulting in unwanted trenches 110 and 112, as well as undercut in the spacer layer (100), as diagrammatically illustrated in FIG. 1b. The unwanted trenches in the spacer layer can be translated to the subsequent process or processes, such as the process for making the mirror plate of the micromirror. As a consequence, the unwanted trenches resulted from the DUV process causes curvature in the mirror plate, which may yield degraded performance of the device or even device failure.
Therefore, what is desired is a method of etching process for use in DUV lithography in making microstructures or semiconductor devices.